Gas-filled springs have a high initial force relative to their maximum force, the initial force being the force that is needed in order for the piston rod of the gas-filled spring to start moving and for springing to commence. In a sheet-metal forming tool a number of gas-filled springs are usually arranged in parallel, so that in sheet metal forming they are acted upon simultaneously by the movements from a press. The stresses in the press and the sheet-metal forming tool will therefore increase rapidly when the gas-filled springs arranged in parallel are simultaneously encountered by the movement of the press. Rapid changes in the forces also occur when the gas-filled springs are simultaneously relieved. These rapid variations in the force place heavy stresses on tools, gas-filled springs and mechanical parts of the press, and also give rise to vibrations.
When the gas-filled spring is placed under a load by the sheet-metal forming tool striking against the piston rod, mechanical noise and rumbling occur. This noise may be experienced as an irritation and may have an adverse effect on the working environment. This applies particularly in industries with many presses and sheet-metal forming tools in the same premises. The noise level varies, depending among other things on which type of press tools and what size of gas-filled springs are used.
The design of sheet-metal forming tools is largely influenced by the intended shape of the sheet metal after forming and the larger the sheet-metal forming tool the more expensive it is. This means that the space in a sheet-metal forming tool is limited by design and cost considerations.
Sheet-metal forming tools and associated parts have high reliability requirements and the speeds for upward and downward movement of the press, for example, may be 0.3 to 1.6 m/s with frequencies of 0.3 to 1.2 strokes per second, an initial force on each gas-filled spring of 7,500 to 180,000 N during a working shift and a total of approximately 1 million strokes, that is to say forming operations between major services.